1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface-emitting laser (VECSEL) apparatus, and more particularly, to a VECSEL apparatus having an improved structure to reduce the incidence loss of the pumping beam.
2. Description of the Related Art
A vertical cavity surface-emitting laser (VCSEL) emits a very narrow spectrum during single longitudinal operation and has high coupling efficiency since its projection angle is small. Other apparatuses can be easily integrated with the VCSEL due to the surface emitting structure of the VCSEL. Thus, VCSELs can be used for pumping laser diodes (LDs).
However, the width of an emission region of the VCSEL must be less than 10 μm for a general horizontal operation of the VCSEL. Even then, since the VCSEL can be easily changed into a multiple mode due to a thermal lens effect according to an increased light output, the maximum output generally is not greater than 5 mW during a single longitudinal operation.
A vertical external cavity surface-emitting laser (VECSEL) device has been suggested to enhance the above-described advantages of the VCSEL and realize high output. In the VECSEL, a gain region can be increased by replacing an upper distributed Bragg reflector (DBR) layer with an external mirror, and an output of 100 mW or more can be obtained. Recently, because it is difficult to obtain sufficient gain in a surface-emitting laser due to small gain volume compared to an edge emitting laser, a VECSEL device having a periodic gain structure in which quantum wells are periodically placed has been developed. Also, as it is limited to uniformly inject carriers into a large area by electric pumping, a VECSEL device has been developed in which a large area is pumped uniformly with carriers by optical pumping in order to obtain high output.
FIG. 1 is a schematic cross-sectional view of a conventional end pumping VECSEL apparatus. FIG. 2 is a graph of the reflectivity of a DBR layer according to the wavelength of the pumping beams in the VECSEL apparatus of FIG. 1.
Referring to FIG. 1, the conventional VECSEL includes a transparent substrate 2, a DBR layer 4 stacked on the transparent substrate 2, a periodic gain layer 6 stacked on the DBR layer 4, an optical pump 9 installed to radiate a pumping beam to the transparent substrate, and an external cavity mirror 8 installed to face the periodic gain layer 6. The DBR layer 4 has a stack structure of H, L, H, L, H, L . . . or L, H, L, H, L, H . . . Here, L denotes a low refractive index layer having a wavelength of λ/4, and H denotes a high refractive index layer, and λ is a wavelength of the laser light generated in the periodic gain layer 6.
In the conventional VECSEL apparatus having the above described configuration, more than 30% of the pumping beam light incident at an interface or on a surface of the DBR layer 4 is reflected and thus the incidence pumping efficiency in the gain region is relatively low, that is, about 70%. Referring to FIG. 2, when a pumping beam has a wavelength of 808 nm, about 30% of the pumping beam light is reflected at an interface of the DBR layer 4. Thus, reflection of the incident pumping beam at an interface of the DBR layer 4 reduces the gain efficiency and the lasing efficiency. Accordingly, a VECSEL apparatus having an improved structure to reduce the incidence loss to increase the pumping beam efficiency is required.